Short-wave communication is a long-distance communication method with long history, which realizes long-distance communication through ionospheric reflection. Since the performance of the ionosphere varies with time, space and radio frequency, causing amplitude fading and phase fluctuation of the signal, it will seriously affect the quality of short-wave communication. At the same time, sky-wave reflection has serious multipath effect, which also causes frequency selective fading and multipath. Delay is the main limitation of data transmission on short-wave links. In addition, the short-wave band is available for a narrow frequency band, small communication capacity, and severe atmospheric and industrial radio noise interference, which greatly limits the development of short-wave communication. Since the 1960s, satellite communications have replaced many important services that were originally short-wave, because of their channel stability, good communication quality, and large capacity. The investment in short-wave communication has been drastically reduced, and its status has been greatly reduced.
However, compared with satellite communication, optical cable and other communication means, short-wave communication can realize long-distance communication without establishing a relay station, and has its own characteristics, such as short construction period and low maintenance cost; simple equipment, easy to conceal; flexible use, circuit It is easy to dispatch, convenient for temporary networking, and strong against damage. These remarkable advantages are unmatched by other means of communication. It turns out that satellite communications, which were supposed to replace short-wave communications, do not meet the needs of users in all situations. Since the 1980s, short-wave communication has received renewed attention due to considerations such as satellite security, and many countries have increased research and development of short-wave communication technologies.
In recent years, due to the rapid development of electronic technology, the short-wave communication technology and equipment have been upgraded. The original shortcomings have been overcome to varying degrees, and the communication quality has been greatly improved. New short-wave communication technologies and systems have been formed, and short-wave communication has been formed. Is moving towards revival. Among them, the most important and significant technological advancement is short-wave adaptive technology.
Shortwave adaptive communication conceptShort-wave communication mainly relies on sky wave, and the ionospheric reflection channel is a kind of time-discrete channel, which is characterized by path loss, delay spread, noise and interference, etc., which change with frequency, place, season, day and night, therefore, The operating frequency in short-wave communication cannot be arbitrarily chosen. For a relatively long period of time, the choice of short-wave communication frequency is determined based on the frequency prediction data [1]. However, the characteristics of the ionosphere vary greatly from day to day. The frequency prediction data is based on long-term observation statistics, and the channel parameters in actual communication cannot be reflected in real time. Moreover, long-term forecasts do not consider factors such as multipath effects and radio interference, resulting in actual The quality of shortwave communication is not satisfactory.
Statistics show that even in the worst case of nighttime communication environment, the short-wave band has about 4% of noise-free channels, while about 27% of channel interference is small or no interference at noon [2]. Therefore, avoiding interference in real time and finding a noisy channel with good propagation conditions is the main way to improve the quality of short-wave communication. The key to achieving this goal is to adopt adaptive technology.
The so-called adaptive, is able to continuously measure signal and system changes, automatically change the system structure and parameters, so that the system can adapt to changes in communication conditions and resist human interference. Broadly speaking, short-wave adaptation includes frequency adaptation, power adaptation, transmission rate adaptation, diversity adaptation, adaptive equalization, and adaptive nulling antennas. Since frequency selection and frequency change are the most effective ways to improve the quality of short-wave communication, so-called short-wave adaptive communication refers to frequency adaptation.
Short-wave adaptive communication has experienced two stages of short-wave frequency management and 2G-ALE, and is developing towards 3G-ALE.
Frequency management systemThe short-wave adaptive system must perform the dual task of detecting channel characteristics and interference distribution in real time. The optimal operating frequency provided by the system is the result of measuring and analyzing the data. The technique used to accomplish this task is called real-time channel estimation. "RTCE" technology. The basic method for realizing short-wave adaptation is to use RTCE (RealTImeChannelEvaluaTIon) technology to measure and analyze various channel parameters, and establish a communication link working at the optimal frequency based on comprehensive analysis and calculation results.
The independent channel detection system can form a frequency management grid in a certain area, and quickly scan and detect the frequency in a short wave range, and obtain a frequency ordering table with good communication quality. Then, according to the needs, it is uniformly distributed to each short-wave communication user in the area. The essence is to provide real-time frequency forecasts for users in the region. Both the US CURTS system and the real-time frequency selection system developed in China can provide a frequency table to the user every 10 minutes [3], and the user selects the best communication frequency in actual communication.
According to different technologies, RTCE can be divided into ionospheric pulse detection, ionospheric frequency modulated continuous wave detection (Chirp), pilot detection, 8FSK signal detection, etc. Among them, 8FSK detection is the most widely used signal format for adaptive radio stations. .
The CURTS system is the earliest real-time frequency selective system that can measure five channel parameters. It uses ionospheric pulse detection. Due to the detection pulse power of up to 30 kW, it will cause serious interference and can only be used in large-area strategic communication systems. In the mid-1970s, Barry of the United States developed the AN/TRQ-35(V) real-time frequency-selective tactical frequency management system using Chirp detection, and later upgraded to AN/TRQ-42(V) in the Gulf of the early 1990s. In the war, these two frequency management systems successfully supported the short-wave communication network and played a key communication guarantee role for the victory of the Allies.
The short-wave frequency management system detection results can reflect the frequency resources of the entire short-wave frequency band and have been sold as commercial software. The short-wave single-station positioning function of some radio monitoring stations is also realized by using these detection results and then calculating. The frequency management system is characterized by separation of communication and detection, and the detection equipment is expensive. This development process is also called the 1G-ALE stage of short-wave adaptive technology.
2G-ALE communication systemWith the continuous development of microprocessor and digital signal processing technology, in the mid-1980s, short-wave adaptive stations that directly used RTCE technology in the communication system to detect, evaluate and communicate short-wave channels appeared. This kind of radio station can select the best short-wave communication channel in real time, reduce the influence of time-varying, multi-path and noise on the short-wave channel, so that the short-wave communication frequency changes with the channel condition, thus ensuring the communication is always in the best quality. Good on the channel. Thanks to the high-speed DSP chip, RTCE is an embedded component of the communication device, which greatly reduces the cost and makes the operation very convenient.
In order to make short-wave adaptive radio intercommunication and networking, in October 1988, the US military issued the military standard MIL-STD-188/141A for short-wave adaptive communication; in 1990, the corresponding federal standard FED-STD-1045 protocol also Formally introduced, the agreement, also referred to as the 1045 agreement, has become the de facto international standard. Short-wave adaptive stations that comply with the 1045 protocol are generally referred to as 2G-ALE products. There are many models of 2G-ALE products, and the functions are similar. The typical equipments are RF-3200, 7100 series, and ALIS radio stations in Germany.
The 2G-ALE adaptive communication system has the following four basic functions.
(1) RTCE function
The RTCE function is called Link Quality Analysis (LQA) in short-wave adaptive communication systems. In order to simplify the equipment and reduce the cost, the general LQA is performed before or during the communication, and is only performed on a limited short-wave channel, usually 10 to 20. The obtained data is stored in the LQA matrix. In actual communication, the system selects the preferred operating frequency according to the order of the channels in the LQA matrix.
(2) Automatic scan receiving function
In order to receive the selective call and perform the LQA test, all the stations in the network have an automatic scan receiving function, which can cyclically scan on a predetermined number of channels, waiting for the call signal or the LQA detection signal.
(3) Automatically establish link function
According to the LQA matrix, the system should be able to automatically establish a communication link. This function is called automatic link establishment ALE (AutomaTIcLinkEstablishment). This is the ultimate problem of short-wave adaptive communication, which is based on the results of accepting automatic scanning, selective calling and LQA comprehensive application. This is the biggest difference between 2G-ALE and 1G-ALE systems.
(4) Channel automatic switching function
Random interference, selective fading, multipath, etc. existing in the short-wave channel may deteriorate the quality of the established channel and may even interrupt the communication. Therefore, the short wave adaptive communication system generally has a channel automatic switching function. That is, in the communication process, when the radio wave propagation condition is deteriorated or seriously interfered, the adaptive system can switch the channel, and the communication frequency is automatically adjusted to the sub-optimal frequency in the LQA matrix.
The 8FSK commonly used in short-wave daily monitoring is one of the most widely used signal formats in 2G-ALE products, and is the LQA detection signal in the channel. Due to the wide application of the 2G-ALE system, the 8FSK signal appears most frequently during monitoring. If some stations have been transmitting 8FSK signals for a long time, it can be preliminarily determined that the communication backbone of a large short-wave communication network is Perform LQA detection.
The 2G-ALE procedure stipulates that each single tone of 8FSK represents 3 bits of binary data (Gray code), and its correspondence is shown in Table 1.
According to the requirements of the 2G-ALE procedure, when the station receives the command or data information, it first converts it into the original frame consisting of the basic ALE word, then performs grouping, Gray coding, interleaving and triple redundancy, and finally performs 8FSK modulation. The signal is sent at a rate of 125 tones per second, so the transmission rate is 375 bit/s and the symbol rate is 125 Baud/s. The phase between the individual tones is continuous, and the transition should be at the maximum or minimum of the waveform (the slope is zero), which ensures that the baseband audio signal occupies the narrowest band and the energy is more concentrated. The actual monitoring of the demodulated 8FSK signal waveform is shown in Figure 1.
3G-ALE communication systemThe 2G-ALE system defined by the 1045 protocol can form an interactive short-wave communication system with strong anti-destructive and simple equipment, which initially meets the needs of users. However, with the development of technology and network, the 1045 protocol also exposes some shortcomings, mainly because it cannot provide an effective link access mechanism; it does not support Internet protocols and applications; there are only two LQA measurement parameters, and the transmission rate is greater than 2400 bit/s. Not enough; the ALE signal requires three handshaking to establish a link, and the connection speed is slow.
In 1999, the US military issued the 3G-ALE military standard (MIL-STD-188/141B), a short-wave adaptive automatic communication network standard. Under the premise of fully supporting the voice communication and small network defined by the second generation agreement, the standard effectively supports large-scale, data-intensive, fast and high-quality short-wave communication systems, once again triggering research on short-wave communication worldwide. climax. In China, relevant research work has also started.
The 3G-ALE fully automatic short-wave network is essentially a wireless packet switching network, which adopts the seven-layer structure model of OSI. The definition and meaning of the lower three layers are shown in Table 2.
Compared with 2G-ALE system, 3G-ALE system has made a lot of improvements: link establishment protocol management (3G-ALE) and data link service management (TM), high-speed data link management (HDL), low-speed data link Management (LDL), Circuit Connection Management (CLM) and other protocols form an interdependent 3G-ALE protocol family, forming a relatively complete new short-wave communication system. 3G-ALE also adopts many new technologies, mainly digital PSK modulation and demodulation method, burst wave BW series waveform transmission, call channel synchronization scanning, intra-network station division into different resident groups, channel separation, time slot access mode. , carrier sense mechanism, etc. The main technical features of the 3G-ALE system are:
(1) Waveform
3G-ALE link establishment and data transmission adopt unified 8PSK modulation, carrier frequency is 1800Hz, signaling rate is 2400B, different uses correspond to different signal formats, and burst wave BW (BurstWaveform) is used to improve system flexibility. . 3G-ALE defines five kinds of burst waves, as listed in Table 3.
BW0 is a 3G-ALE data protocol unit, which is similar to the 8FSK of the 2G-ALE system. The total frame length is 1472 symbols, wherein the frame preamble sequence length is 640 symbols octal sequence, and the original information field 26 bits passes the code rate 1. /2FEC encoding, interleaving, Walsh spreading, and then adding to the fixed PN code sequence modulo 8 to form an information octal sequence of 832 symbols, together with the preamble to form a 3G-ALE frame.
It can be seen from Table 3 that 3G-ALE uses the orthogonal Walsh function for spreading, and adopts Rake receiving technology to realize multipath diversity, thereby greatly improving anti-interference and anti-fading performance. Except for the BW4 used by LDL, all waveforms use FEC forward error correction code, which greatly simplifies the adaptive algorithm and improves the channel pass rate. In the case of LDL, the enhanced ARQ protocol can be used to ensure minimum communication capabilities.
(2) Channel separation
The 3G-ALE system separates the call channel from the data stream channel and keeps the data channel adjacent to the call channel so that they are consistent in transmission characteristics, which facilitates the monitoring of the transmission channel and ensures high efficiency of information transmission. The rapidity of link establishment.
(3) Synchronous link
3G-ALE provides two asynchronous and synchronous link establishment modes, especially the synchronous mode, with smaller delay and better reflecting the characteristics of 3G-ALE. In this way, the calling party sends a call, and the called party sends a response signal after receiving the call. When the calling party receives the response signal within the specified time, the two parties establish a connection, otherwise the link establishment attempt fails.
(4) Residing group division
The DwellGroup concept is introduced in the 3G-ALE system to divide all the stations in the network into multiple groups. At the same time, the stations in the unified resident group work on the same channel, while the stations in different groups work differently. On the channel, this can greatly reduce system congestion. The calling station clearly knows the channel where the destination station is located, reducing the channel residence time of the station.
(5) Division of time slots
To reduce channel congestion, 3G-ALE also adopts a slotted slot technology. The 3G-ALE station has a dwell time of 4 s on one channel, and 3G-ALE divides it into 5 time slots, 800 ms per time slot. The first time slot is used for tuning and monitoring, to determine whether there is communication traffic, which is convenient for the next communication; the remaining four time slots are collectively referred to as call and response time slots for the transmission of protocol data units.
3G-ALE automatic short-wave network equipment includes radio, ALE controller and ALEModem, data controller and data modem, network controller HFNC and so on. Due to its outstanding technical features, its performance has been greatly improved. It can be realized: fast link establishment can reach 1.6s at the earliest, and a successful link only needs to complete two-way transmission, which greatly reduces the link-building time and ALE information in the air exposure time; reliable minimum communication capability, extremely low-speed chain-building capability, can reach -20dB, the minimum acceptance of data packets can reach 95% [4], and has anti-continuous wave, Anti-burst interference capability; full network synchronization work, support up to 1920 sites and greater information, priority channel access and anti-collision measures; support Internet protocols and applications.
Since the 3G-ALE radio signals all use 8PSK, it is very important to monitor and analyze the 8PSK signals. However, since the identification and analysis of the PSK signal is much more complicated than the FSK signal, most of the monitoring devices can demodulate the PSK signal, but the post-analysis and processing software is not intuitive and mature. Therefore, the monitoring of 3G-ALE stations can only be carried out using traditional methods.
ConclusionFrequency adaptive communication technology is the basis of modern short-wave communication, and many new short-wave communication technologies are related to frequency adaptation. With the application of 3G-ALE, modern short-wave communication systems adopt more new technologies and have better performance [5]. In terms of channel technology, frequency adaptive technology is still developing. Technologies such as spread spectrum and frequency hopping have entered the practical stage. CHESS systems with hopping speeds up to 5000H/s are being developed. In terms of terminal technology, OFDM technology can be used in shortwave. The transmission rate of 16kbit/s to 64kbit/s is realized on the channel; the software radio technology will make the short-wave communication have a more open structure and flexible performance. All of this shows that short-wave communication, like other information technologies, has entered a period of rapid development and has become an important technical support for the information society.
The rapid development of short-wave communication poses a serious challenge to the radio management and monitoring departments. China's radio short-wave monitoring network has only been built for a few years, and the performance and quantity of equipment are far from meeting the needs. The experience and level of personnel need to be improved [6]. In the face of the new situation of the development of short-wave communications, we must catch up and catch up with each other in order to successfully complete the tasks assigned by the state.
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